XWIKI-11312: Horizontal white line when using search suggest with Database search configured
* We need a flag to know that we are in multi-source mode because the list of sources is flatten so we cannot distinguish between single-source mode and multi-source mode with only one source activated/enabled.
* Even if there are no sources configured/activated/enabled we should still display the 'No results' message.
* In order to support an empty source we introduce a way to define a source using a function with a callback, instead of an URL.